Dragon, my girls can be too rough from time to time. Sometimes playful tickling or head rubbing becomes way too rough and Max cries. The 5-year-old is much better at playing gently, but my 3-year-old seems to be really conflicted. It's like, she loves him SO much, but every once in a while she gets a spike of jealousy (she was the baby too) and almost unintentionally makes Max uncomfortable. She never hurts him though, like the pinching. I would certainly intervene if that happened. It's weird that their parents don't say anything. I would talk to the parents, since ultimately they should be helping teach the kids. I don't think little ones that age are intentionally mean, it's just that they have no filter for emotions like jealousy and it's a great opportunity for the kids to learn. That said, if the parents did nothing I would step in and protect the babies.
Sierra, Max was 16 pounds at his 4-month check-up, so I imagine about 17 or 18 now? He's quite a chunk, though. I don't worry too much about percentiles, because I was SUPER fat as a baby... like off-the-charts fat. Once I started walking it all worked out. DH was tiny as a baby, and is now...ahem...a bit rotund. If your ped isn't concerned about Camden's weight, I wouldn't worry (easier said than done!).